Traveling wave tube



March 16, 1954 J. w. TILEY 2 ,672,572

TRAVELING WAVE TUBE Filed Nov. 28, 1947 3 Sheets-Sheet l INVENTOR. JOAM/W 11512.)

March 16, 1954 J. W. TILEY TRAVELING WAVE TUBE Filed Nov. 28, 1947Poms-.9

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ATTORNE Ye? March 16, 1954 Filed Nov. 28, 1947 J. W. TILEY TRAVELINGWAVE TUBE 5 Sheets=$heet 3 II I .33

IXI'EX'I'OK JOHN W TILEY Ja/ZLQ BY Patented Mar. 16, 1954 UNITED STATESPATENT OFFICE.

TRAVELING WAVE TUBE John W. Tiley, Philadelphia, Pa., assignor to PhilcoCorporation, Philadelphia, Pa., a corporation of PennsylvaniaApplication November 28, 1947, Serial No. 788,724

9 Claims. 1

This invention relates in general to the electron tube art, and moreparticularly to a novel electromagnetic traveling wave tube havingspecial application in high frequency and broad band electrical systems.This application is a continuation-in-part of my co-pending applicationSerial No. 761,798 filed July 18, 1947, now Patent No. 2,541,843.

In electrical systems such as television, pulseposition modulation andradar, it has often been extremely diificult to obtain adequate anduniform amplification over the frequency spectrum encompassed by thesignal. Ordinary triodes, even when incorporated in special circuits,fail to provide a usable gain in such applications. Recent designs, suchas the lighthouse tube, and the velocity modulation tube as exemplifiedby the klystron, can provide reasonable gain solely in narrow band widthoperation. An attempt to use a klystron or like tube in the microwaveregion with a band width of fifty megacycles or greater will resultgenerally in a gain of less than unity.

A recent electronic development, known as the beam traveling wave tube,overcomes the limitations of the more conventional tube types, and hasbeen successfully tested as an efficient amplifier of signals having amean frequency of the order of thousands of megacycles with an over-allband width of the order of eight hundred megacycles. Theoretically, evenmuch wider bands may be amplified by the device without sacrifice ofgain.

For a description and illustration of this development, reference ismade-to the publication Bell Laboratories Record of December, 1946, andto the article therein entitled The Beam Traveling Wave Tube by J. R.Pierce. In this reference the traveling wave tube is described asconstituted of an electron gun similar to those employed in cathode raytubes. An electron beam generated by the gun is directed in a narrowbeam along the axis of a long evacuated tube and impinged upon acollector anode. Within the tube, and surrounding the beam axis, is aclosely wound wire helix which is excited at the electron gun endthereof by the weak signal to be amplified and which provides at thecollector end thereof the amplified output signal. The tube contains nosignal grids in the conventional sense.

Broadly speaking, the applied signal travels along the wire helix as anelectromagnetic wave at a speed approaching the speed of light. As isdetermined by the pitch of the helix, the Wave 2 travels axially of thetube at a fraction of the speed of light; and the electron gun andcollector anode potentials are arranged so that the average axialvelocity of the electron beam through the helix is somewhat greater thanthe axial wave velocity.

Interaction of the electron beam and electromagnetic fleld componentsextending from the helix produces the signal amplification. The greaterthe e-ectron current and the longer the helix, the greater is the gain.In transit through, the helix, the average electron velocity is reduced,and the energy drop represented by this decreased velocity is impartedto the signal. The tube does not require a tuned circuit in the signalpath, the wire helix being in effect an all pass transmission line.Hence the tube is capable of operation over an exceedingly widefrequency range. In practice this range is limited somewhat by theimpedance match of the helix to the external circuits.

In operation the signal appearing on the helix acts on the electronstream and gradually produces fluctuations in velocity and density. Thedensity modulated electron beam delivers energy to the wave and over thehelix section nearest the collector there is a substantially uniformgain per unit length of travel.

Th present invention contemplates and has as a primary object theprovision of electromagnetic traveling wave tubes of novel and improveddesign having wide application as broad band microwave amplifiers,oscillators, frequency convertors, or the like. As in the electron tubesdisclosed in my above-identified patent application, the electromagnetictraveling wave tubes of the present invention differ from prior arttraveling wave tubes in that wave guides are utilized for directing wavesignals around the path of an electron beam. These wave guides permitdirect coupling of the electromagnetic fields therein and the tubeelectron beam, and preclude excessive energy radiation and undesirablecoupling with external fields.

In the aforementioned patent application it was demonstrated that signalamplification could be obtained in an electromagnetic traveling wavetube by causing an electron beam to traverse the axis of first andsecond or input and output helical wave guide sections of predeterminedpitch and number of turns. The input wave guide was energized by theweak incoming signal, whereby the electron beam was bunched or densitymodulated. The modulated beam was then caused to traverse a second oroutput helical wave guide section and to deliver energy thereto toprovide the amplified output signal.

In a tube of this construction, input and output waveguides are spacedaxially of the electron beam. When certain conditions of beam velocityand helix structure are properly satisfied, considerable gain isobtained and the electron tubes are capable of either amplifier oroscillator operation; the latter being accomplished by coup-ling apredetermined portion of the output signal in the correct phase to theinput wave guide. In connection with traveling wave tubes utilizingindependent input and output wave guide sections, it has been determinedthat the optimum average beam velocity through the guides is equal tothe axial component of velocity of the electromagnetic traveling wave.

In accordance with the principles of the present invention a singlehelical wave guide is utilized for both modulating and extracting energyfrom an electron beam. The single wave guide employed is constructedessentially the same as the input wave guide of the systems disclosed inthe above named patent application.

A particular advantage of the present construe tion to be described indetail below is that the single wave guide utilized for electron beammodulation and signal output comprises a conductive helix of uniformpitch. The incoming signal is coupled into the wave guide at the inputor electron gun end of the helical wave guide and the output signal isderived from the opposite end thereof.

Successful operation as amplifier or oscillator is accomplished by theutilization of an electron beam velocity somewhat above the axialvelocity of the traveling wave. In transit through the tube each groupof electrons delivers energ to the traveling wave and as a consequencethereof, the output signal is at a levelconsiderably higher than thatintroduced at the input end of the guide.

It is therefore an object of the present invention to provide abeam-traveling wave electron tube characterized by inherent stabilityand high gain, and which utilizes a single helical Wave guide structurefor directing the wave energy about the path of the electron beam.

A further object of thepresent invention is to provide a velocitymodulation type electron tube utilizing a single helical wave guide ofuniform pitch for raising theintensity level of an input electromagneticsignal.

A still further object of the present invention is to provide atraveling Wave electron tube of comparativeh simple mechanical designand rugged overall construction.

It has been observed that frequency variations of the signal beingamplified tend to afiect the strength of the electric fields establishedby the wave guide and coupling with the electron beam. The presentinvention further contemplates the provision of novel helical wave guidestructures which materially increase the frequency range over which waveguide electromagnetic traveling wave tubes may operate. Generallyspeaking, this is accomplished by capacitrxely loading the wave guidestructure to mainta n the strength of electric fields more nearlyconstant over wide band frequency variations of the inputelectromagnetic energy.

It is thus another object of the present invention to provide a heli'ialwave guide structure adaptable to many types of traveling wave tubes,

4 which structure permits broad band operation thereof.

Still another object of the present invention is to provide means forcapacitively loading a wave guide having an open side wall whilepermitting the fields extending from the wave guide to couple with anelectron beam.

These and other objects of the present invention will now becomeapparent from the following detailed specification when taken inconnection with the accompanying drawings in which Figure 1 is a generalside view partially in section of a helical wave guide traveling waveelectron tube.

Figure 2 is a general side view partially in section-of anotherembodiment of a traveling wave tube of the general type illustrated inFigure 1.

Figure 3 is a fragmentary side view partially in section of a helicalwave guide tube structure illustrating a novel means of obtaining broadband wave guide operation; and

Figure 4 is a general side view partially in section of a wave guidestructure similar in features and characteristics to that illustrated inFigure 3.

ith reference now to the drawings and more particularly to Figure 1,there is illustrated an electron tube incorporating the features of thepresent invention and comprising, generally, a centrally disposed sealedand evacuated glass or similar dielectric cylinder 2 I Sealed into theleft end of the glass cylinder .21 as viewed in Figure l are theelectrodes of an electron gun 2%, similar to those utilized inconventional cathode ray tube structures. As shown the electron gun 2?]comprises a heater 22 and its associated cathode 23, a centrallyperforated control grid 2 and focusing and accelerating hollowcylindrical electrodes 25 and 26, respectively. When the electrodescomprising the electron gun 29 are energized from a suitable powersource (not shown) an axial beam of electrons of predetermined highvelocity is generated and directed along the axis 28 of cylinder 2|toward a disc shaped collector electrode 2'! sealed into the righthandend of the tube as viewed in Figure 1.

Structural means required for supporting the electron gun electrodeshave been omitted for clarity.

As electron beam tubes of the type included within the structure of thetube in Figure l are sufficiently well known in the electronic art, afurther description thereof and of the energizing means therefor areconsidered unnecessary at this point.

In accordance with the broad principles of the present invention, theaxial path of travel of the electron beam through tube 2! issubstantially enclosed within a single helical wave guide structure 39,preferably of uniform pitch. As is clearly illustrated in Figure 1, thewave guide structure 35? is comprised of a thin highly conductivemetallic strip 3i wound edgewise about and in contacting relationshipwith cylinder 2i at the aforementioned uniform pitch.

The outer edge of the conductive strip 3! defines a helix lyingin'electrical contacting relation with the inner cylindrical surface ofan enclosin: holow metallic or otherwise conductive cylinder 33, coaxialwith and extending substantially the length of tube 2!. The helicalstrip 3! and enclosing metallic cylinder 33 thereby define a helicalpassage 32 of uniform rectangular cross-section which progresses axiallyof tube 2 i. In ef'ect, there "ore, helical passage 32 comprises ahelical wave guide enclosed at any point by two adjacent turns of strip3|, and by the metallic surface of the enclosing cylinder 33. Theinnermost wall of the helical passage 32 lies in the outer surface ofdielectric cylinder 2 l, and is accordingly open insofar as radiation ofelectromagnetic fields is concerned.

Coupling means are provided for the interchange of electromagnetic waveenergy with the helical wave guide passage 32. As illustrated, the outercylindrical conductor 40 of a coaxial line 39 extends through aperforation 4| in metal cylinder 33 and enters helical passage 32. Theend of inner conductor 42 of coaxial line 39 is curved as shown andelectrically connected to the outer conductor 40 to form a smallcoupling loop 43. A similar coaxial line 44, comprising an outerconductor 45 and an inner conductor 46, extends through a perforation 41in cylinder 33 and is terminated by coupling loop 43 within wave guide32. It will be noted that coaxial lines 39 and 44 enter wave guidepassage 32 at axially opposed ends thereof.

In order to maintain electrons traversing cylinder 2| within a sharplydefined axial beam, means are provided for establishing a substan tiallyuniform and unvarying axial magnetic field. As illustrated, this isaccomplished by means ofa multi-layer solenoid type coil 5| coaxial withtube 2| and wound upon an insulating cylindrical coil form 52 suitablysecured over the metallic cylinder 33 between coaxial connectors 39 and44. Coil 5| is preferably energized from a direct current source (notshown in the draw ings).

In accordance with well understood principles, an electron which tendsto travel a path divergent from the axis 28 of tube 2 and henceangularly of the field established by coil 5|, will be urged back toaxis 28' as a consequence of its interaction with the axial magneticfield.

For successful operation of the electron tube structure of Figure 1, itis desirable to preclude possible interference from strayelectromagnetic fields and, further, to preclude internal wavereflections. Insofar as external electric fields are concerned, theconductive cylinder 33 and the helical strip 3| provide substantiallycomplete shielding. A magnetic shield may be employed if foundnecessary.

To minimize internal signal reflections, helical wave guide structure 30has been suitably electrically terminated and matched at the endsthereof. Thus, in the embodiment of Figure 1, impedance matching iseffected at ends of the wave guide by means of two uniformly tapered,molded helical blocks 54. Blocks 54 are preferably constituted of aceramic containing powdered graphite as the dissipating element;however, various other absorptive substances may be employed. By thustapering the lossy material, a proper impedance match is readilyobtained. This corresponds in effect with the utilization of tapered orwedge-shaped dissipative blocks as reflectionless terminations forconventional rectangular and other wave guides.

To insure reflectionless termination, each of the blocks 54 illustratedin Figure 1 is preferably made equal to at least one wave length at thelowest frequency of operation, as measured along the helix. Suitabletermination may also be effected by uniformly tapering the spacing ofthe end turns of wave guide structure 3|) to zero, and covering thetapering sections thereof with a power absorbing, semi-conductivesubstance.

The operation of the traveling wave electron tube illustrated in Figure1 will now be described when effective as a signal amplifier. Theincoming weak signal to be amplified is applied to the tube throughcoaxial line 39 and the amplified output is extracted through coaxialline 44. The input signal travels in a helical path through passage 32at a velocity somewhat less than that of light. Standing waves areprecluded by the impedance matching dissipative blocks 54.

In accordance with the general principles of traveling wave electrontube operation, the velocity component of electromagnetic wave energywithin wave guide 32 parallel to the axis 28 of tube 2|, is a fractionof the velocity of light, as determined by the pitch of the wave guide32. As an example, if the length of the helical path through wave guidepassage 32 is ten times as great as the axial distance encompassedthereby, then the velocity of wave propagation considered relative tothe axis of tube 2| will be equal to one-tenth the velocity of light. Itis therefore apparent that if an electron in the beam generated by thetube electron gun 23 has an average axial velocity equal to one-tenththe velocity of light, then the relative phase of the electromagneticwave in helical passage 32 and this electron will remain unchanged.

If the electron velocities in the beam traversing the tube are of theorder of one-tenth the velocity of light or less, then the followingnonrelativistic equation may be utilized as a guide for determining theelectron velocity:

v=5.93 10 /fi centimeters per second wherein: Y

E is equal to the voltage through which the beam electrons areaccelerated.

The interaction of an electromagnetic wave traveling within helical waveguide 32 and an electron beam traversing the axis 28 of glass cylinder2| is somewhat similar to the bunching action obtained in velocitymodulation electron tubes. As will hereinbelow be demonstrated, the waveenergy traveling within the wave guide 32 from the input coaxial line 39to the output coaxial line 44 continuously density modulates the axialelectron beam in accordance with the input signal variation so that theelectron beam as it traverses the axis of tube 2| is progressivelybunched to greater and greater bunch densities.

Simultaneously with the bunching effect of the electron beam, the beamdelivers energy into wave guide 32 so that the wave energy thereinprogressively increases in intensity in its travel toward collectorelectrode 21.

The electromagnetic fields established by wave energy flowing within thehelical wave guide passage 32 may best be visualized if the regionbetween the inner surface of conductive cylinder 33 and the axis of tube2| is considered as half a rectangular wave guide, the broad walls ofwhich are the metallic surfaces of adjacent turns of helical strip 3|.If the coupling loop 43, applying the input signal to be amplified tothe helical wave guide 32, excites this wave guide in the T1301 mode,then maximum electric field is produced between adjacent inner edges ofthe turns of helical strip 3|. The electric field is normal to the strip3| and diminishes sinusoidally to zero at the inner surface of metalliccylinder 33.

Points A, B, C and D may be considered as defining the cross-section ofhalf a rectangular wave guide operating in the aforementioned mode.

Aspoints C and D are .metallically-connected, there .is no potentialagradient therebetween. However, an electric field is establishedbetween points A and Bbyrt-heexcitingwave, which field cyclicallyvariesin intensity as afunction of time from axpredetermined maximum inone direction through zerotoa corresponding maximumin the oppositedirection. The electric field between points .A :and. B, of course,extends through the dielectric cylinder 2i and into the regionof'electronnflowtherethrough. ;In view of the factthat the :input.electromagnetic wave constantly :progrosses from left,to:right;as-viewed in Figure 1, agivenipotential gradient may beconsidered as traveling around theinneriedges of the helical strip i 3l'ata velocity approaching the velocity .of light. 2Thus,\if a voltage:maximum in aparticular-zdirection appears between points :Aand ;B atuoneinstant, it will appear at correspondingly later intervals of timebetween-points E and It,

B and G, .F and'l-Liand so on, until down the guide.

For purposes of illustration, let it be assumed t'hatthe dimensions of.wave guide 32 are such that, electromagnetic wave energy, introduced bycoupling .loop 43, travels around one complete turn or the helicalpassage 32in a time period corresponding to one-half.cycle of the inputwave. Underlsuchconditions there is a sinusoidal voltagedistribution-between opposed inner edges of the'helical strip'3l. .If,,under these conditions, andiana particular instant of time, the voltagebetween pointsArandB-is a maximum in one direction, then the potentialdifference between points E-andl w'illbezero and between points Y B andG a maximum in the opposite direction.

Within the glass cylinder 2!, the electricfiel'ds extending :betweenopposed points on adjacent turns of the helical strip: 3! may beresolved into components, one of which is parallel to the axis 28 ofelectron flow. "The sense of these parallel field-components isdependent upon the-instantaneous polarity of the originating points. Aswave energy passes from left to right through the helical 'wave guidepassage '32, these field com- "ponentsmay be considered as progressingaxially from left' to right, while simultaneously rotating "about thetube axis. As previously mentioned, the axial velocity oi'propogation orthe electromagnetic field components will be determined 'primarily'bythe pitch of the helical-guide '32.

If'the electron gun potentials were to be adjuste'd to provide-anaxialelectron beam having a velocitysubstantially equal to the axialvelocity of propagation of theinput'electromagnetic wave (asdescribedintheaforementioned patent application and herein discussed forillustrative purposes only), a bunching action will take place ass.result 'ofthe interaction of the traveling electronbeamand the axiallymoving electric field-components. l'n a given electron beam, individualelectron velocities extend over aspectrum from a velocity considerablybelow to one considerably'above the average velocity. If at thetimeelectrons enterthat portion-of the axis ponent in its .travelbetween points ;:A :an'tisB, :;E and F, B and G, and the like. 'O-nztheother hand, electrons which enterztub 2 I theireg-ion of helical guidestructure 30 with a tvelocity greater than the axial velocity of'thetraveling wave, will be retardedsomewhatand will T6011- tinue todecrease in velocity intransitth-rough the tube.

Electrons which enter the region of the helical wave guide 30 when pointAis positive'withrespect topoint B, will be retarded. Of these retardedelectrons, the ones having the highest velocities may escape theretarding held and enter into a region of anaccelerating field; of-thetype previously described. The lowest velocity electrons may besuificiently retarded lid-cause them to be overtaken by the followingcycle-of an accelerating fieldcomponent.

Thus, as the electron beam traverses the section of tube 2! spanned bythe helical wave guide structure 30, it is acted upon by the travelingwave field components so that electron 'willbecome progressively bunchedin the region ofi max imum positive potential points of the electricfield. Th degree of bunching, ordensity mod- .ulation, depends ofcourseupon the intensityci the input electromagnetic wave and the'effective length of the helical wave guide passage 32. .The densitymodulated beam which is .formed, -as hereinabove described, .finallyimpinges upon collector plate 2?.

Since the electron beam is pr.ogressively..rednced in velocity asittravels. throughthe region of helical wav guide 32, it is preferablethat.,,the collector electrode 2'? flbeconnecteduto a source ofpotential (not shown) which is thatltequirfidlio produce such electronvelocity. as determinedby the above equation.

In order for the electronbeam travelingwave tube illustrated inFigurelto amplify electromagnetic wave ener y, applied overcoaxialcohector til, it is essentialthat therebea nettransfer of energy from theelectron beam totthei'elem tromagnetic wave traveling Withinthe waveguide passage'32. From the foregoing discussion1it'is apparent that toaccomplishthis ener y exchange it is necessary that the electronbeamactually enter the region spannedbythewaveguidestructure 30 With anaverage electron velocity somewhat greater than theaX-ialvelocityoitheelectric field components which extend from ,the open walloi guideztflinto theregionofthe electron beam.

As in the aboveexarnpla-electrons enteringsa region of positivepotential gradient will-berati- .celerated in the direction-pi thecollector-electrode 2'! while thoseventeringga region sir-negativepotential gradientwill be retarded. j .How- -ever,.-since in operationthe electrons en.ter;the region of wave guide struotureeilxwithan-average velocity greater than the axialyelocity of the axial fieldcomponents electronswill remain in a region of retarding field for alonger-period than in a region of accelerating field. At-agiven instant,therefore, a-greater number of electrons are being retarded thanaccelerated, whereby the amount of energy given up to the travelingwa-veby the electrons at a give-n'instant will be greater than the amount ofenergy absorbed by the electron beam from the traveling wave. 'Thus asthe electromagnetic wave travels. its helical path from the inputcoaxialline39 to the output coaxial line 44, the intensity of the waveenergy therein progressively,buildslup, resultingin a net amplificationfor the system, the amplified signal being extracted at coaxial lin 54.

Clearly the actual gain obtained by a tub of the type illustrated inFigure 1 is dependent upon numerous factors including the length of thehelical wave guide 32 between input and output connectors, the actualvelocity differential between the average electron velocity and theaxial velocity of the traveling wave, The precise design factorsinvolved in the construction of a tube of this type will not be treatedin greater detail in the present application.

The inter-action of th electric field component extending into thecylinder 2! in the region of the electron beam may also be explained byconsidering the axially moving electrons as encountering electric fieldswhose magnitudes are continuously increasing as the electrons traversethe cylinder 2! from the electron gun 29 to the collector anode 21.

When considered in this manner, each retarding field will effectively begreater in intensity than the previous accelerating field with theresult that electron bunching will occur in the regions of retardingfield. As a phenomenon of this type results in the transfer of energyfrom the beam to the wave, energy will, be delivered by the electrons tothe electric field to increase further the amplitude of the travelingwave energy. Since the electron velocity in the beam when considered onan average is greater than that of the axial velocity of the travelingelectric fields, the electron bunches will not continuously be comprisedof the same individual electrons; rather electrons will drift betweenelectron bunches.

The electron tube illustrated in Figure 1 has a particular tendency tobecome unstable or act as an oscillator unless considerable care isgiven to the matter of preventing reflections at both ends of thestructure. When employed as an ampliher, as in the above description ofFig. 1, it is particularly important to terminate properly and match thewave uide 32 to the external circuits.

On the other hand. if it is desired to operate the electron tubeillustrated in Figure l as an oscillator for the generation of highfrequency electromagnetic wave energy, a feedback circuit deliveringenergy from the coaxial line A l to the input terminal of coaxial line39 of proper phase may be provided. If the gain of the system iscomparatively high, then merely mismatching the terminations at in utand output connectors will result in self-oscillation at a naturalfrequency which may be determined by placing suitable frequencysensitive means in the energy input and output coupling systems.

Referring now' to Figure 2, there is illustrated a microwave amplifieror oscillator having the general electrical features illustrated anddescribed in connection with Figure 1, but differing.

somewhat in construction. The electron tube of Figure 2 comprises aglass, or similar dielectric cylinder 6!, sealed and evacuated to permitthe passage of an electron beam therethrough. Sealed into the left-handend of cylinder 6|, as viewed in Figure 2, is an electron gun comprisingan indirectly heated cathode 52, a perforated disc-type grid 63, andfocusing and accelerating cylindrical electrodes 64 and 65,respectively. As

described in connection with Figure 1, when theseelectrodes areenergized from a suitable power source (not shown), an electron beam isgenerated having an average electron velocity determined as a functionof the total accelerating 16 potential in accordance with the equationset forth above.

The generated electron beam is directed along axis 56 toward a collectorelectrode 87, suitably sealed into the right-hand end of the cylinder6|. For amplifier and oscillator operation, the potential of collectorelectrode 6'! is determined in a manner similar to that describedhereinabove for collector electrode 21 shown in Figure 1.

In accordance with the principles of the present invention, a helicalwave guide H is coaxially disposed with respect to the glass cylinder 6|and positioned between the ends thereof. Helical wave guide H ispreferably formed of a coil of unitary U-shaped metallic channelcomprising side walls 73 and M rigidly spaced by integral outer metallicstrip '55. As illustrated the inner open face of this U-shaped channelis formed to lie in the outer surface of cylinder 6|.

In order to prevent signal reflections and stand-- ing waves within thewave guide H, helical dissipative blocks 18 and 18' are secured withinthe ends of this wave guide. As described in connection with theimpedance matching blocks 54 of Figure 1, blocks 18 and 18' arepreferably uniformly tapered and constituted of a material capable ofeiiectively dissipating microwave energy. The length of a block 78 whenmeasured helically about the guide is greater than a wave length at thelowest frequency of tube operation.

Electromagnetic wave energy is introduced into the wave guide H througha coaxial line 16, the

outer wall of which enters the wave guide section through a perforationH in the outer metallic strip l5. A loop 8| from the coaxial line innerconductor provides ample signal coupling. in a corresponding manner acoaxial cable 82 is utilized to extract signal energy from the waveguide H. Thus the outer conductor of coaxial cable 82 extends into thewave guide H through a perforation 83 therein, and a loop 84 formed ofthe inner conductor serves as the coupling means.

The outer helical surface of the wave guide H is substantially enclosedwithin a Bakelite or similar insulating cylinder 85, upon which auniform, solenoid-type coil 86 is wound. When coil 86 is energized froma direct current source (not shown), a magnetic field is establishedhaving a large component parallel to the axis 65 of electron travel.This magnetic field minimizes de-. focusing of the electron beam in itscomparatively long axial transit through the cylinder 6! between theelectron gun and the collector electrodev Complete structural means forrigidly positioning the elements of the electron tube of Figure 2- inthe relation shown have not been illustrated.

, The operation of the electron tube illustratedin Figure 2 isessentially similar to that alreadyanode 6?. The actual axial velocityof the travel-;-

ing wave is a fraction of the velocity through the wave guide H due tothe helical path traversed therethrough.

Dissipative blocks 78 and 18' ensure the the axial direction of electronflow. Between any two opposed points, such as M and N of the open sideof the helical wave guide ll, an electricficld is established with acomponent within the ab-- sence of traveling waves in a direction otherthan 1'1" tube Erl parallel to the axis 85 of the'electronfiovw Thiselectric field component may be visualized as spinning about the axis ofthe tube ill at somewhat lessthan the velocity of light as the wavetravels'irom left to right as viewed in Figure 2.

Theaverage velocity of electrons entering'the space within tube filencompassed by the helical wave guide ll is greater than the axialvelocity of the traveling wave. As described above the electron beam isdensity modulated as it traverses the region of the helical wave guidetoward the collector anode til. In View ofthe velocity relationshipsspecified for the axial component of the traveling wave and the electronbeam, the bunching operationdescribed is accompanied by a generalretardation of the beam electrons with the result that energy isdelivered from the electron beam to the traveling wave.

As the traveling wave thus approachegthe col lector anode 6-1, theenergy level thereof is greatly increased whereby theenergy'delivered'to coaxial cable 82 is of considerably higher levelthan that introduced at coaxial cable it.

As a result of the separation of individual turns of helical wave guideii, there are regions as for example between points N and P within thecylin-- der 6! ofcomparatively uniformly weal: electric field; In otherwords, the electric field is at all times concentrated between opposedbroad walls I3 and IA-0f the waveguide. In effect, therefore, regionswithin the cylinder 6!, such as that between points N and P, act asdrift spac s through which the velocity-of electrons remainssubstantially unaffected.

The electron tube of Figure 2'may be connected for oscillator operationby connecting a portion of the signal output obtained in coaxial cable82 through proper phasing means to the input coaxial cable 16L circuitof suitable form (not shown) is preferably connected into the feedbackmeans.

As in the case of the electron tube illustrated in Figure l, the beamtraveling wave tube of Figure 2 is substantially unaffected by signalfrequency changes between wide limits or by the -2 bandwidth thereof. Itis preferable that the diameter of cylinder 6! be chosen such that whenthe volume enclosed therebyis considered as a cylindrical wave guide, ithas a cut-oil frequency above the range of operation of the tube,whereby direct wave propagation and wave reflections at the signalfrequency tending to distort the normal electric fields are precluded.

Inter-action with the electron beam of wave energy in the wave guideinter-turn space 33 is highly undesirable. By filling the inter-turnspaces 88 by an absorptive material (not shown) the eflect'of inter-turnenergy may be substantially eliminated. Although an inner-turn spacedissipative material has not been illustrated in 1 Figure 2 ofthe'present application, there is illustration and discussion thereof inaforemen-- tioned co-pending patent application.

The electron tubes illustrated in Figures 1 and- 2 of the presentapplication are capable of operation' as microwave amplifiers oroscillators overextremely wide bands. However, it has been ob servedthatfrequency variations alter the strength of the electric fieldsestablished between opposed points such as M and'N of Figure 2' of thehelical wave guide.

The range of operation'of the electron tubes of Figures 1 and 2 may beextended considerablyin frequency by incorporating wave guideconstructions' as illustrated in Figures 3 and 4.

For stability of operation, a filter acra-wa Referring now to Figure 3',there is illustrated. a portion of an electron tube generallyconstructed in accordance with theprinciples illustratedin Figure l, butmodified to include a frequencybroadening means.

Thus in Figure 3 there is shown a dielectric cylinder 21 surrounded byan edge-wound helical strip 3i of conductive material which forms aguide 30 extending" helical channel or wave through th electron tube.The edge-wound strip 31 is wholly enclosed within a metallic cylinder 33which in turn is covered by a Bakelite coilform 52 for the solenoidalcoil 55. The coinponents of Figure 3, namely members 2!, Si, 33,

5| and 52, function as the correspondingly desig-- nated elements of theelectron tube of Figural.

The sole modification incorporated in the structure of Figure 3 is theaddition of a flat helical metallic strip lfil.

strip 3! which defines the sides of helical Wave guide 333. Further, thehelical gap 562 defined by eling wave within the wave guid 38 in turnestab- 35 lishes an electric field directed across the helical gap $62,which field is in cilect similar to the field described asformed betweenpoints 5 and'G of the electron tube of Figure l with the exception of.increased intensity.

The'field extending acrcssithe gap I02 also'extends through dielectriccylinder 21 and spins helically as it progresses down the guide betweenthe inputand output couplersinot shown in Figure 3).

Electrically the capacitiveloading introduced by the helical strip Silltends to maintain the strength of the electric field existing across thegap substantially constant for wide frequency variations of the inputwave energy. As a consequence of this construction, the traveling Waveelectron tube of Figure 3 is operable over. a wider frequency band. thanthe corresponding tube shown in .Figure l. The application of a helicalstripsuch as it?! thus provides a certain design freedom whenconstructing the wave guide structure" 39.

Referring now toFigure l, thereis illustrated in fragmentaryforma'modification of the electron tube shown in Figure 2 correspondingessentially to the. above described modification of the elec-- tron tubeof Figure .l as illustrated in Figure 3.

In Figure 4 the dielectric cylinder 6 l, the helical wave guide l l, theenclosing Bakelite coil form 85, and the solenoid-condo correspond tothe similarly designated components of the electron tube of Figure 2.Other structural features of Figure 2- have been omitted as they areunessential to the description of th present modification.

To increase the frequency band of operation possible for a beam electrontube of this type, a helically wound flat metallic strip l l i has beenpositionedupon the outer surface of cylinder 81, and

has apitch corresponding to that of helical wave Theaxia'lwidth of thestrip i l l is such that; at this pitch, there-is provided a uniform Thestrip Hit as illustrated in Figure 3 is of a pitch equal to that of thehelical helical air gap I I2 between adjacent turns thereof, similar toair gap I02 described in connection with Figure 3.

The air gap I I2 which is of helical form is arranged as illustrated inFigure 4 as to lie centralling of the open inner wall of the U-shapedwave guide section II. It may also be seen that the helical strip Illcompletely spans the interturn space 88 between adjacent turns of thewave guide ll.

In operation, traveling wave energy appearing within wave guide Hestablishes a particularly intense electric field across the gap H2which functions in the manner already described in connection with thetravelin fields of Figures 1 and 2, with the advantage that theconstruction illustrated is operable overa much greater frequency range.

From the foregoing description of the present traveling wave electrontubes, it is apparent that these offer design possibilities unattainablewith other known means. Although described above as functioning assignal amplifiers, these tubes may be employed as microwave modulatorsand detectors, and in other circuits where conventional tube types failto give the desired performance.

Numerous modifications oi the apparatus herein disclosed are possiblewithout departing from the spirit of the present invention. Asdescribed, a helical path is employed for signal propagation to reducethe axial velocity of wave propagation to the average electron velocity.A large pitch, necessitating fewer turns for the input and output waveguides, may be used if the wave guides are filled with a solid, low lossdielectric substance. This is possible since the velocity of wavepropagation in a solid dielectric is substantially less than thevelocity in free space.

In order to increase further the energy output of the tubes illustrated,the helical wave guides may be operated with a uniform increasingpotential gradient. Thus, the guides may be formed of a resistivematerial rather than of highly conductive substances as hereinabovedescribed, and

a direct voltage maintained between the ends thereof, the positive endbeing furthest along in the direction of electron travel. This positivegradient will accelerate and add to the kinetic energy of the electrons,and permit the extraction I of increased power from the bunched beam.

It will now also be obvious to those skilled in the art that theinventive principle herein disclosed may, without departure from theinvention, take the form in which both the wave guide and electron beamsystem are enclosed in a common evacuated envelope as illustrated inFigure 3.

of the parent ap lication of this application, Serial No. 761,798 filedJuly 18, 1947.

In view of the many possible structural and electrical designmodifications possible, it is preferred that the present invention bedefined solely by the appended claims.

I claim:

1. An electron tube comprising an evacuated dielectric cylinder, meansfor generating an electron beam therein, a helical wave guide formed ofchannel shaped conductor having substantially rectangular cross-section,a flat helical conductive strip formed in engagement with the outersurface of said dielectric cylinder, the open end of said channelengaging said strip, said strip being formed with adjacent turns spacedfrom each other so as to effect a helical gap of pitch equal to thepitch of said helical wave guide, said gap being disposed centrally ofsaid open end of said wave guide and permitting energy radiation fromsaid wave guide into the region of said elec-. tron beam, and means forintroducing energy to and extracting energy from said wave guide.

2. An electron tube comprising an evacuated dielectric cylinder, meansfor generating an electron beam therein, a helical wave guide formed ofchannel shaped conductor having substantially rectangular cross-section,a fiat helical conductive strip formed in engagement with the outersurface of said dielectric cylinder, the open end of said channelengaging said strip, said strip being formed with adjacent turns spacedfrom each other so as to efiect a helical gap of pitch equal to thepitch of said helical wave guide, said gap being disposed centrally ofsaid open end of said wave guide and permitting energy radiation fromsaid wave guide into the region of said electron beam, and means forintroducing energy to and extracting energy from said wave guide, saidwave guide being formed to permit inter-action of electromagnetic waveenergy flowing therein with said electron beam.

3. An electron tube comprising an evacuated dielectric cylinder, meansfor generating an electron beam therein, means for directin said beamalong a predetermined path, a continuous helical wave guide formed ofchannel shaped conductor having substantially rectangular cross-section,a

flat helical conductive strip formed in engagement with the outersurface of said dielectric cylinder, the open end of said channelengaging said strip, said strip being formed with adjacent turns spacedfrom each other so as to effect a helical gap of pitch equal to thepitch of said helical wave guide, said gap being disposed centrally ofsaid open end of said wave guide and permitting energy radiation fromsaid wave guide into the region of said electron beam, and wave energycoupling means associated with each end of said wave guide.

4. An electron tube comprising an evacuated dielectric cylinder, meansfor generating an electron beam therein, means for directing saidelectron beam along a predetermined path, and a continuous helical waveguide formed of channel shaped conductor having substantiallyrectangular cross-section, a flat helical conductive strip formed inengagement with the outer surface of said dielectric cylinder, the openend of said channel engaging said strip, said strip being formed withadjacent turns spaced from each other so as to effect a helical gap ofpitch equal to the pitch of said helical wave guide, saidgap beingdisposed centrally of said open end of said- Wave guide and permittingenergy radiation from said Wave guide into the region of said electronbeam, said wave guide being formed to permit inter-action ofelectromagnetic wave energy flowing therein with said electron beam, andsaid wave guide having energy input and output couplings displacedlongitudinally of said path.

5. An electron tube comprising an evacuated dielectric cylinder, meansfor generating an electron beam therein, means for directing saidelectron beam over a substantially linear path, a wave guide ofsubstantially rectangular cross-secticn helically wound around said beampath, a flat helical conductive strip formed in engagement with theouter surface of said dielectric cylinder, the open end of said guideengaging said strip, said strip being formed with adjacent turns spacedfrom each other so as to effect a helical gap of pitch equal to thepitch of said helical; wave. guide, said;- gap being; disposed conetrally of saidopen. endofsaid; wave: guide and permitting; energyradiation iromlsaid wave guide into-the regional saidelectron-beam, saidwave guide being; adaptedto permit density modulationof said electronbeam. in accordance with electric wave energy flowing therein; and toex!- tractenergy from. a: density modulated,- electron beam;

6. A beamtraveling wave electrontubeoomprising. in combination: an.evacuated dielectric cylinder, an electron. gun disposed at an endofsaid.- cylinder forgenerating an electron beam of predetermined averageelectron. velocity, a collector electrode positionedwithin the opposedend 01 said dielectric-cylinder, means iordirecting said generatedelectron" beam axially through'said dielectric cylinder and impingingsaid .beamupon said: collector electrode, a helical. Wave guide formedof obannelshaoed conductor, having substantially rectangularcross-section, a fiat :helical conductive strip formed in-engagementwiththe outer surface of soididielectric cylinder, theopen end of saidchannel engaging said strip; said strip; being formed with adjacentturns spaced from each other so as: toefiect. a helical gap of pitchequal to the pitch of-said helical Wave guide, saidgap being disposedcentrallyof said open .end of said a wave guide and permitting energy.radiationfrom said waveguide into the region or said electron beam, andmeans for introducing and extracting energy fromisaid wave guide;

7. An; electron. tube comprising an evacuated dielectric cylinder,means'ior generating an; elec tronbeanrtherein, a helical waveguide-ofsubstantially rectangularrcross-section. having one of itsconductive-bounding: surface in engagement with. the, outer: surface ofsaid: dielectric oy-lindcn, said: one bounding: surface beingprovidedwith: a can runningglengthwise thereof and, following thehelicalkconvolutions otsaid surface; said gap being disposed centrallyof: said one bouncingsurface and. permitting energy radiation from said:waveguide into the. region: of, said electronbeam; and means for,introducing energy to and extracting energyfrom'said waveguide, the:axial dimension. of said gap, being: less than the axial dimension ofthecross section ofrsaid' wave guide.,

8; A beam". traveling'wavc': electron.-. tube: comprising: incombination an evacuated: dielectric cylinder; an e ectron gun disposed:atan end of said: cylinder for. generating an; electron beam ofpredetermined average: electronz velocity; a col-'- leotorv electrodepositioned within the: opposed and of said dielectriccylinder;meansiondirect ing'said. generated electronibeam: axially through said.dielectric cylinder. and impinging saidbeam uponsaid; collectorelectrode, a helical Waveguide of, substantially rectangularcrosssectionhaving one-- at its. conductive bounding surfaces: in: engagement with the outer surface of said choice-.- tric cylinder, saidone bounding surface bein provided with a gap running lengthwise thereoiand. following the helical convolutions of said surface, said gap beingdisposed. centrally of said one bounding surface and permitting energyradi ation from saidwaveguide into the region of said. electron beam,and means for introducing energy toiand extracting energy from said.Waveguide, the axialdimensionof said'gap being less than the axialdimension of the cross-section ofsaid waveguide.

Q. A beam traveling Wave electron tubeicom prising in. combinationevacuated dielectric cylinder, an electron gun,v disposed at aniendofsaid cylinder for generating an electron beamof predetermined averageelectron velocity, acollector electrode positioned Within. the opposedend of said dielectriccylinder, means for direct ing said generatedelectron beam axially through said-dielectric cylinder and impingingsaidbeam upon said collector electrode, a helical Waveguide ofsubstantially rectangularcross-section having one of its conductivebounding surfaces in one gagement with the outer surface of said diBlectrio cylinder, said one bounding surface; being provided with. a gaprunning lengthwise thereof? and following the helical convolutions ofsaid surface, said gap being disposed centrally-ofsaid one boundingsurface and permitting energy radiation from .said waveguide-into theregion of'said electron beam, and, means for introducingenergy; to andextracting energy'from said waveguide; said Waveguide being adapted topermit density modulation oi S8.-id':10l31011 beam in accordance; withelectricvvave energy flowing therein,.and-to. extract energy fromadensity modulatedelectron beam, the axialdimension ofsaid gapbeing lessthan the axial dimensionof thecross-section of;- said wave guide.

JOHN W. TILEY.

References Cited inthefile of this patent UNITED STATES PATENTS NumberName Date 2,300,052 Lindenblad Oct. 27, 1942' 2,367,295 LlewellynJan..16, 1945 2,368,031 Llewellyn Jan. 23, 194:5 2,413,608 Di Toro Dec.31, 1946 2,439,401 Smith Apr. 13, 1948. 2,578,434 Lindenblad Dec. 11,1951" OTHER REFERENCES.

Article: by: Pierce, Bell Lab. Record, December-

